This invention relates to radiant energy collection and more particularly to a cavity design for reducing inherent losses in certain radiant energy collectors.
This invention relates to the subject matter of my copending patent application, Ser. No. 113,155, filed Jan. 18, 1980, now U.S. Pat. No. 4,359,265; assigned to the same assignee.
It is well-established that radiation which is partially collimated with an angular divergence of .+-..theta. can be concentrated on a receiver without transmission loss by the maximum concentration of C.sub.max =sin .theta..sup.-1 through the use of nonimaging reflectors known generically in the art as compound parabolic concentrators. These concentrators are disclosed in my U.S. Pat. Nos. 3,923,381; 4,002,499; and 4,003,638 the disclosures of which are incorporated herein by reference. The compound parabolic concentrator (CPC) includes an energy receiver positioned between two trough-like sidewalls which reflect substantially all incident energy received over a predetermined included angle (.+-..theta.) onto the energy receiver. The aforementioned patents teach that the profile curve of at least portions of the reflective sidewalls should be concavely shaped to assume the maximum possible slope consistent with reflecting maximum angle energy rays onto an energy receiver. In addition, U.S. Pat. No. 4,002,499 teaches, for a tubular energy receiver, that the lowermost portions of the sidewalls should form an involute of the shape of the energy receiver to maximize energy collection.
While the energy receiver shown in the '499 patent is the theoretical "optical design" receiver, practical designs dictate that a larger-than-theoretical receiver shape be emplaced between the sidewalls to ensure that placement tolerances, minor wall malformations, etc. do not significantly hinder the energy concentrator. More recently, it has been shown desirable to encompass the receiver in a vacuum bottle-like structure wherein the outermost surface is concentric to the inner receiver; is transparent; and encloses a vacuum between itself and the inner surface of the receiver (which itself may be coated with an energy selective surface).
To strictly meet the requirements of maximum concentration and no transmission loss in a CPC, the reflector surfaces should touch the optical design receiver. At times, this is not practical--especially when a larger-than-theoretical receiver is employed or when the receiver is enclosed in a transparent vacuum jacket.
In fact, it is desirable in most constructions that the actual receiver be offset somewhat from the reflector walls to prevent mechanical interferences when they are assembled. This offset reduces the efficiency of the system.
W. R. McIntire of the Argonne National Laboratory has developed a cavity/reflector design for the region below a tubular absorber. The McIntire cavity, for gaps between the cavity and the absorber of up to approximately half of the optical receiver radius, eliminates some of the losses referred to above. That design is shown in FIG. 1 and utilizes a trough-shaped cavity 10 positioned beneath the optical receiver 12. The "W" design (four linear segments) of cavity 10 ensures that no energy rays can enter the region between receiver tube 12 and cavity 10 without being reflected onto receiver 12. R.sub.1 is the radius of the circular cross section of the optical receiver, and R.sub.2 is the maximum distance which cavity 10 can be separated from the center of optical receiver 12. McIntire shows that for the four-segment cavity 10, R.sub.2 should be no larger than .fwdarw.2R.sub.1. McIntire also shows that when his "W" cavity is combined with a CPC concentrating structure, its capability to concentrate energy is somewhat degraded in comparison to the cusp-type CPC concentrator shown in U.S. Pat. No. 4,002,499.
One problem with the four-segment cavity 10, when it is combined with a CPC, is the complexity of manufacture of the structure. In other words, the more "bends" that are required in a mirrored surface, the more difficult it is to maintain the structural rigidity, optimum wall shape, and reflectivity of the trough.
It is, therefore, an object of this invention to provide an optical trough cavity which provides enhanced energy concentration ability.
It is another object of this invention to provide an optical reflecting cavity of simple construction which may be used in conjunction with a CPC structure.